1. Field of the Invention
The present invention relates to a solid-state image sensor for producing image signals representative of an optical image such as a color still picture formed on an array of photosensitive cells thereof, and a digital camera using the same.
2. Description of the Background Art
Today, a digital camera loaded with a CCD (Charge Coupled Device), a MOS (Metal Oxide Semiconductor) or similar solid-state image sensor is extensively used. The number of pixels available with such an image sensor has recently become great enough to implement high-definition color pictures. On the other hand, the individual photosensitive cell included in the solid-state image sensor for photoelectric conversion is decreasing in size in parallel with an increase in pixel density and a decrease in the overall size of the camera and for the purpose of increasing yield. A decrease in cell size directly translates into a decrease in pixel size and therefore a decrease in sensitivity to light for a single cell. Moreover, to produce a color picture, primary-color or complementary-color filter segments are positioned in front of photodiodes, which constitute the cells. Each color filter segment reduces the quantity of light to reach the associated photodiode due to its spectral transmission characteristic. Consequently, a sufficient amount of information is not available when the camera picks up a scene in a low illumination environment.
Digital cameras may include image-shooting adjustment features such as AE (Automatic Exposure) and AF (Automatic Focus) controls by using image signals output from an image sensor. Japanese patent laid-open publication No. 161078/1993, for example, discloses an electronic still camera with a CCD image sensor configured to assign different shutter speeds between the odd- and even-numbered fields constituting a single frame. The resulting two photometric values are used to determine the lightness of a subject for thereby effecting AE control.
Also, the camera disclosed in the above prior art document has an AF control function for calculating an estimated contrast value on the basis of high frequency components separated from image signals. The camera then moves a lens included therein to a position where contrast is higher for thereby executing AF control. Further, the camera amplifies the level of each image signal with a variable gain in accordance with the signal level to thereby correct a luminance level and adjust white balance.
However, a problem with the conventional digital camera of the type described is that when the sensitivity of the cells is short, noise components relatively increase and make it difficult to accurately execute the pickup control. For example, when the camera picks up a scene around dark, at night or in a room, it is difficult for the camera to attain image signals of a luminance level high enough for accurate image-shooting adjustment due to the short illumination of the scene. In such a case, it has been customary to produce pixel signals in mixture in order to increase the quantity of information conveyed by the image signals, thereby promoting accurate image-shooting adjustment.
The conventional digital camera of the type described additionally has a function of compensating for defective pixels particular to the image sensor to thereby generate corrected image data. Japanese patent laid-open publication No. 203465/1995, for example, teaches a signal interpolation for interpolating defective signals derived from defective pixels, which are included in a single-chip color solid-state image sensor. Further, Japanese patent laid-open publication No. 262025/1999 proposes a picture interpolation for correcting the signals of defective pixels with pixel signals of the same color as the defective pixels.
However, the mixed-pixel read-out scheme needs the same transfer rate as the whole-pixel read-out scheme and is therefore lower in pixel transfer rate than skipped-pixel read-out scheme. The mixed-pixel read-out scheme therefore fails to smoothly display a movie on a monitor, e.g. before an actual shot. For example, with an image sensor having 1,280*1,024 effective pixels in which the mixed-pixel read-out is carried out with each four or eight pixels reduced to one, a pixel reading time is necessary as long as about 1/7.5 second or 1/15 second, respectively. The pixel output rate further decreases with an increase in pixel density.
With a digital camera having the one-to-eight pixel skipping scheme, in which all the pixels are read out so that every eight pixels are mixed into one, pixels eight times as many as the usual pixel skipping scheme have to be read out even in the one-to-eight mixed-pixel skipping mode. Although those pixels convey information eight times as much as the latter, the one-to-eight mixed-pixel skipping scheme needs the same read-out time as the whole-pixel read-out scheme. Moreover, even with the one-to-eight mixed-pixel skipping scheme, the required amount of information is not always available.
Under the above circumstances, it has been difficult to realize accurate AE and AF control without lowering the output rate in a dark place short of illumination. Particularly, when the illumination of a desired scene is low, noise components contained in the image signals relatively increase and thus lower the accuracy of control information for image-shooting adjustment.
Moreover, a color filter positioned on the array of photosensitive cells of a solid-state color image sensor lowers sensitivity by about one-third in the case of an R (red), G (green) and B (blue) or primary-color filter, compared to an image sensor without a color filter. Even a complementary-color filter lowers sensitivity by about one-half. The color filter therefore further obstructs accurate image-shooting adjustment in a poor illumination environment.